64kB intros, 64k for short, are like demos but with an added arbitrary limitation on the size: they must fit entirely within a single binary file of no more than 65536 bytes. No extra assets, no network, no extra libraries: the usual rule is that it should run on a freshly installed Windows PC with up to date drivers.
This is crazy.
http://www.pouet.net/prod.php?which=1221
“This is Crazy” Is it? Then check out 4kb.